Electromagnetic wave interference (EMI) or radio frequency interference (RFI) results from the operation of various electronic devices. Many devices such as electronic memories are extremely sensitive to EMI or RFI pollution which can deprogram memories and has the potential for ruining the circuitry and thus rendering the device useless. Historically this was not a problem to industry because of metallic housings that acted as a shield and the fact that sensitive electronic devices were not in wide use. The use of plastics in place of metal for the manufacture of housings for electronic devices results in a structure that allows electromagnetic waves to flow virtually unimpeded through the device. The Federal Communications Commission has established regulations to limit the amount of electromagnetic radiation that may be emitted by an electronic device (FCC Docket No. 20780). This regulation divides computing devices into two broad categories of those intended for business (Class A) and those for home use (Class B). The limits are defined by an electromagnetic wave having a given field strength or less at a given distance throughout a given frequency range. The industry has met the standard imposed by the FCC regulations by using vacuum metalizing or silver filled paints which are quite costly.
Other industry standards for these products are VDE German standard 0871/6.78; the proposed SAE AIR 1499; and U.S. Department of Health MDS-201-004.
Many solvent based coatings have been used for this reason and they have included electrically conductive additives such as nickel, copper and silver. The term plastic is used to refer to those polymeric materials commonly used for housing for electronic apparatus such as polyphenylene ethers, polycarbonates, polyacrylates, polysulfones, polystyrenes, flame retardant polystyrenes, epoxy resins and the like.
The applicants have discovered a method for making a coating composition containing electrically conductive powder that results in a coating composition that adheres to plastic substrates and is capable of shielding sensitive electronic components from electromagnetic radiation.
Thus, the primary object of the present invention is to provide a composition that is capable of blocking electromagnetic radiation and also a coating composition that adheres to and does not cause the plastic substrates to become damaged by solvents.